1. Field of the Invention
The present invention relates to reduction of metal oxides in an electrolytic cell.
2. Description of Related Art
The present invention was made during the course of an on-going research project on the electrolytic reduction of titania (TiO2) carried out by the applicant.
During the course of the research project the applicant carried out experimental work on an electrolytic cell that included a graphite crucible that formed an anode of the cell, a pool of molten CaCl2-based electrolyte in the crucible, and a cathode that included solid titania.
One objective of the experimental work was to reproduce the results reported in International application PCT/GB99/01781 (Publication no. WO99/64638) in the name of Cambridge University Technical Services Limited and in technical papers published by the inventors.
The Cambridge International application discloses two potential applications of a xe2x80x9cdiscoveryxe2x80x9d in the field of metallurgical electrochemistry.
One application is the direct production of a metal from a metal oxide.
In the context of this application, the xe2x80x9cdiscoveryxe2x80x9d is the realisation that an electrolytic cell can be used to ionise oxygen contained in a metal oxide so that the oxygen dissolves in an electrolyte. The Cambridge International application discloses that when a suitable potential is applied to an electrolytic cell with a metal oxide as a cathode, a reaction occurs whereby oxygen is ionised and is subsequently able to dissolve in the electrolyte of the cell.
European patent application 9995507.1 derived from the Cambridge International application has been allowed by the European Patent Office.
The allowed claims of the European patent application inter alia define a method of electrolytically reducing a metal oxide (such as titania) that includes operating an electrolytic cell at a potential that is lower than the deposition potential of cations in the electrolyte.
The Cambridge European patent application does not define what is meant by deposition potential and does not include any specific examples that provide values of the deposition potential for particular cations.
However, submissions dated Oct. 2, 2001 to the European Patent Office by the Cambridge patent attorneys, which pre-dated the lodgement of the claims that were ultimately allowed, indicate that they believe that the decomposition potential of an electrolyte is the deposition potential of a cation in the electrolyte.
Specifically, page 5 of the submissions state that: xe2x80x9cThe second advantage described above is achieved in part through carrying out the claimed invention below the decomposition potential of the electrolyte. If higher potentials are used then, as noted in D1 and D2, the cation in the electrolyte deposits on the metal or semi-metal compound. In the example of D1, this leads to calcium deposition and therefore consumption of this reactive metal . . . . During operation of the method, the electrolytic cation is not deposited on the cathodexe2x80x9d.
Contrary to the findings of Cambridge, the experimental work carried out by the applicant has established that it is essential that the electrolytic cell be operated at a potential that is above the potential at which Ca++ cations in the electrolyte can deposit as Ca metal on the cathode.
Accordingly, the present invention provides a method of reducing a metal oxide in a solid state in an electrolytic cell, which electrolytic cell includes an anode, a cathode formed at least in part from the metal oxide, and a molten electrolyte, the electrolyte including cations of a metal that is capable of chemically reducing the cathode metal oxide, and which method includes a step of operating the cell at a potential that is above a potential at which cations of the metal that is capable of chemically reducing the cathode metal oxide deposit as the metal on the cathode, whereby the metal chemically reduces the cathode metal oxide.
The applicant does not have a clear understanding of the electrolytic cell mechanism at this stage. Nevertheless, whilst not wishing to be bound by the comments in this paragraph, the applicant offers the following comments by way of an outline of a possible cell mechanism. The experimental work carried out by the applicant produced evidence of Ca metal in the electrolyte. The applicant believes that, at least during the early stages of operation of the cell, the Ca metal was the result of electrodeposition of Ca++ cations as Ca metal on electrically conductive sections of the cathode. The experimental work was carried out using a CaCl2-based electrolyte at a cell potential below the decomposition potential of CaCl2. The applicant believes that the initial deposition of Ca metal on the cathode was due to the presence of Ca++ cations and Oxe2x88x92xe2x88x92 anions derived from CaO in the electrolyte. The decomposition potential of CaO is less than the decomposition potential of CaCl2. In this cell mechanism the cell operation is dependent at least during the early stages of cell operation on decomposition of CaO, with Ca++ cations migrating to the cathode and depositing as Ca metal and Oxe2x88x92xe2x88x92 anions migrating to the anode and forming CO and/or CO2 (in a situation in which the anode is a graphite anode). The applicant believes that the Ca metal that deposited on electrically conductive sections of the cathode was deposited predominantly as a separate phase in the early stages of cell operation and thereafter dissolved in the electrolyte and migrated to the vicinity of the titania in the cathode and participated in chemical reduction of titania. The applicant also believes that at later stages of the cell operation part of the Ca metal that deposited on the cathode was deposited directly on partially deoxidised titanium and thereafter participated in chemical reduction of titanium. The applicant also believes that the Oxe2x88x92xe2x88x92 anions, once extracted from the titania, migrated to the anode and reacted with anode carbon and produced CO and/or CO2 and released electrons that facilitated electrolytic deposition of Ca metal on the cathode.
Preferably the metal deposited on the cathode is soluble in the electrolyte and can dissolve in the electrolyte and thereby migrate to the vicinity of the cathode metal oxide.
In a situation in which the metal oxide is a titanium oxide, such as titania, it is preferred that the electrolyte be a CaCl2-based electrolyte that includes CaO as one of the constituents of the electrolyte.
In such a situation it is preferred that the cell potential be above the potential at which Ca metal can deposit on the cathode, i.e. the decomposition potential of CaO.
The decomposition potential of CaO can vary over a considerable range depending on factors such as the composition of the anode, the electrolyte temperature and electrolyte composition.
In a cell containing CaO saturated CaCl2 at 1373K (1100xc2x0 C.) and a graphite anode this would require a minimum cell potential of 1.34V.
It is also preferred that the cell potential be below the potential at which Clxe2x88x92 anions can deposit on the anode and form chlorine gas, i.e. the decomposition potential of CaCl2. 
In a cell containing CaO saturated CaCl2 at 1373K (1100xc2x0 C.) and a graphite anode this would require that the cell potential be less than 3.5V.
The decomposition potential of CaCl2 can vary over a considerable range depending on factors such as the composition of the anode, the electrolyte temperature and electrolyte composition.
For example, a salt containing 80% CaCl2 and 20% KCl at a temperature of 900K (657xc2x0 C.), decomposes to Ca (metal) and Cl2 (gas) above 3.4V and a salt containing 100% CaCl2at 1373K (1100xc2x0 C.) decomposes at 3.0V.
In general terms, in a cell containing CaO-CaCl2 salt (not saturated) at a temperature in the range of 600-1100xc2x0 C. and a graphite anode it is preferred that the cell potential be between 1.3 and 3.5V.
The CaCl2-based electrolyte may be a commercially available source of CaCl2, such as calcium chloride dihydrate, that partially decomposes on heating and produces CaO or otherwise includes CaO.
Alternatively, or in addition, the CaCl2-based electrolyte may include CaCl2 and CaO that are added separately or pre-mixed to form the electrolyte.
It is preferred that the anode be graphite or an inert anode.
The applicant found in the experimental work that there were relatively significant amounts of carbon transferred from the graphite anode to the electrolyte and to a lesser extent, to the titanium produced at the cathode under a wide range of cell operating conditions. Carbon in the titanium is an undesirable contaminant. In addition, carbon transfer was partially responsible for low energy efficiency of the cell. Both problems could present significant barriers to commercialisation of electrolytic reduction technology.
The applicant also found that the dominant mechanism of carbon transfer is electrochemical rather than erosion and that one way of minimising carbon transfer and therefore contamination of titanium produced at the cathode by electrolytic reduction of titania is to position a membrane that is permeable to oxygen anions and is impermeable to carbon in ionic and non-ionic forms between the cathode and the anode and thereby prevent migration of carbon to the cathode.
Accordingly, in order to minimise contamination of titanium produced at the cathode resulting from carbon transfer, it is preferred that the electrolytic cell includes a membrane that is permeable to oxygen anions and is impermeable to carbon in ionic and non-ionic forms positioned between the cathode and the anode to thereby prevent migration of carbon to the cathode.
The membrane may be formed from any suitable material.
Preferably the membrane is formed from a solid electrolyte.
One solid electrolyte tested by the applicant is yttria stabilised zirconia.
According to the present invention there is also provided an electrolytic cell as described above and operating in accordance with the above described method.